1. Introduction
This invention relates to an apparatus for automatically controlling the composition of an electroless plating solution whereby the components of the plating solution are maintained nearly constant during use of the plating solution.
2. Description of the Prior Art
In the past, electroless plating solutions in commercial use have been controlled through manual analysis of the plating solution during use followed by manual addition of plating components as shown to be necessary by analysis. A disadvantage of this procedure is that by the time the analysis is performed and the replenishment requirements calculated, the plating solution, assuming it has been operating continuously, will have undergone further compositional change so that the component levels calculated may be as much as 10% to 20% inaccurate at the time the additions are actually made to the bath. This change in the concentration levels results in inconsistency in deposit characteristics and properties.
If the workload in the plating bath is reasonably constant or can be calculated, it is possible to program the additions of components necessary to replenish a bath so that they can be periodically made with some degree of success. However, it is still necessary to verify the concentrations by analysis at least several times during a working day. To eliminate that time-lag encountered in manual analysis and uncertainty of unprogrammed or periodic additions to a plating solution, attempts have been made to automate the analysis and to control the addition of consumed components by additions of replenishers on a continuous basis. In this respect, using an electroless copper plating solution as an example, it is known that the principal reaction occurring in a plating bath during a plating operation is that reaction represented by the following equation: EQU Cu.sup.++ +2HCHO+4OH.sup.- .fwdarw.Cu.sup.o +H.sub.2 +2H.sub.2 O+2HCOO.sup.-.
As can be seen from the above equation, the consumable ingredients in the plating solution are copper, formaldehyde and hydroxide which react in a definite stochiometric ratio and must be replenished in the same ratio to maintain the composition of the plating solution constant. It would appear that, because of the stoichiometric relationship, monitoring any of these ingredients would provide the necessary information for controlling the replenishment of the three ingredients. In practice, it has been found that there are additional side reactions which take place independently of the main reaction beyond that described above. The most serious of these reactions is the well known Canizzaro reaction where formaldehyde and hydroxide react with each other in accordance with the following equation: EQU 2HCHO+OH.sup.- --CH.sub.3 OH+HCOO.sup.-
From the above equation, it would appear that in addition to copper, formaldehyde and hydroxide should be monitored, but monitoring either could provide a determination of the amount of the unmonitored component through calculation. In practice, this is not the accurate because formaldehyde evaporates from solution and hydroxide reacts with carbon dioxide in the air resulting in additional loss not accounted for by the above equations. Nonetheless, a two-component monitoring control device using copper and hydroxide as a basis for programming a control system is the subject of U.S. Pat. No. 3,532,519, incorporated herein by reference. This patent discloses a method for monitoring copper and hydroxide and further discloses the use of hydroxide content to determine formaldehyde content.
In the method disclosed in the patent, a sample stream from the plating bath is pumped through a colorimeter for copper determination, and through a pH meter for a determination of the pH of the bath. The system of the patent provides for a preselected set-point established by either the colorimeter or the pH indicator, whereby a relay activates an appropriate pump to introduce aqueous alkali hydroxide solution and/or mixed formaldehyde and copper salt solution, until the sample readings taken from the bath again return to normal or the pre-set condition. This method is also summarized in "GALVANOTECHNIK," 61(3), 215 (1970) by W. Immel, also incorporated herein by reference.
The prior art methods disclosed in the aforesaid publications have been found less than satisfactory with modern highly active electroless copper solutions because the device does not account for formaldehyde loss through evaporation and hydroxide loss through reaction with the carbon dioxide in the air. In addition, the copper in such solutions undergo autocatalytic deposition after relatively short periods of operation of the system, producing deposition on the colorimeter walls as well as on the pH electrodes thereby causing inaccurate readings and unreliable functioning of both control systems. Also, the pH of the operating bath is not a reliable indicator of the hydroxide concentrations under the conditions employed, since modern plating solutions operate at a pH of 12.5 or higher where the reading is no longer linear with hydroxide concentration due to buffering and sodium ion interference.
In U.S. Pat. No. 4,096,301, incorporated herein by reference, there is disclosed a method and apparatus for monitoring and adjusting the composition of an electroless copper plating solution which involves withdrawing a sample stream from the running bath and running this through three separate analysis stations. It is disclosed in said patent that an essential part of the analysis procedure utilizes a test acid solution of known, standardized normality introduced at a constant feed rate and mixed into the sample stream to produce an optimum pH level as a datum level for a first analysis. Change in pH from this level is said to provide a signal of hydroxide concentration changes occurring in the plating bath since such change is an analog of the hydroxide content of the working bath. This creates a control for providing for signaling for replenishment of hydroxide to the bath to maintain a desired working level of the component. The acidification is also said to serve the purpose of reducing the sensitivity of the sample solution to autocatalytic degradation resulting in plate-out of metal onto the sensing members of the analyzing instruments in the controller. Absent plate-out on the sensing members, accurate colorimetric readings are obtainable for controlling the replenishment of the copper in the plating bath. Finally, the patent teaches a second pH analysis of the acidified sample stream following addition of a test sulfite solution of known strength and rate of addition. Sulfite reacts with formaldehyde to produce hydroxide ions raising the pH of the sample. This reading is made continuous and a change from a predetermined level is utilized to signal addition to the plating bath of formaldehyde.
Though the invention disclosed in U.S. Pat. No. 4,096,301 is an improvement over the prior art, it has been found in practice that acidification of the sample stream does not completely eliminate autocatalytic degradation and consequently, plate-out does occur on the sensing members, albeit at a slower rate than in prior art control devices. Consequently, the colorimeter is coated with metal affecting its function and its ability to measure copper content. In addition, other lines within the controller become plugged with plated copper after prolonged use of the control device which interferes with its operation. Finally, the system suffers from inaccuracy for failing to account for formaldehyde evaporation and hydroxide losses by reaction with carbon dioxide.